1. Field of the Invention
The present invention relates to both a signal measuring apparatus which measures analog signals such as signals output from a device under test and a semiconductor testing apparatus which includes the signal measuring apparatus.
Priority is claimed on Japanese Patent Application No. 2006-302639, filed Nov. 8, 2006, the content of which is incorporated herein by reference.
2. Description of the Related Art
In conventional cases, a semiconductor testing apparatus such as a memory tester and a logic tester is used in order to test and/or check initial problems of devices under test. Such a semiconductor testing apparatus includes a signal measuring apparatus which provides an analog/digital converter (hereinafter, A/D converter) for converting analog signals output from the device under test to digital signals. The semiconductor testing apparatus conducts predetermined operations on the digital signals converted by the signal measuring apparatus in order to measure the analog signals output from the device under test.
Patent document 1 (Japanese Patent Application, First Publication No. H04-36672) discloses a semiconductor testing apparatus which tests multiple items in parallel with respect to a device under test which inputs and outputs mixed signals including various signals such as a DC signal, an analog signal and a digital signal.
Regarding the above-described semiconductor testing apparatus, it should be noted that there is an aspect in which an upper limit of a performance, ability or accuracy of measuring analog signals is mainly determined by a performance of the A/D converter. Therefore, with regard to the signal measuring apparatus included in the semiconductor testing apparatus which measures with high accuracy, it is necessary to provide an A/D converter with high performance. Here, with regard to the A/D converter which has high performance, there are various requirements such as ability to convert a DC (direct current) component of the analog signals with high accuracy (high DC performance), low noise (high noise performance), low distortion (high distortion performance), and low spurious component (high spurious performance). There are a very small number of the A/D converters which satisfy all of such requirements, and moreover, such A/D converters are very expensive.
For example, there are generally used A/D converters which are a delta-sigma (ΔΣ) A/D converter and a successive approximation A/D converter. In general, the delta-sigma A/D converter has an excellent noise performance, but the delta-sigma A/D converter has large distortions (has a high level at high harmonics), that is, there is a tendency in which the delta-sigma A/D converter has lower distortion performance compared to the successive approximation A/D converter. On the other hand, the successive approximation A/D converter has an excellent distortion performance, but the successive approximation A/D converter generates large noise, that is, there is a tendency in which the successive approximation A/D converter has lower noise performance compared to the delta-sigma A/D converter.
Therefore, in conventional cases, a user selects an A/D converter which has an excellent performance with respect to one point among the above-described requirements which is important for the user and applies the selected A/D converter to the signal measuring apparatus, or the user selects a well-balanced A/D converter which has good performances to some degree with respect to all of the above-described requirements and applies the selected A/D converter to the signal measuring apparatus. Moreover, there is a proposal of a conventional technique in which a band-elimination filter is provided at a former step (input side) of an A/D converter in order to achieve both lower noise and lower distortion, and the user applies the A/D converter to which such a conventional technique is used to the signal measuring apparatus.
Here, the above-described conventional technique applying the band-elimination filter uses characteristics of an A/D converter in which a large distortion is generated if a large voltage is applied to the A/D converter, and decreases the distortion by using the band-elimination filter which removes a fundamental wave component having a large signal level from signals received by the A/D converter in order to decrease a level of input voltage. Moreover, a frequency of the band-elimination filter is measured beforehand, and signals output from the A/D converter are corrected or adjusted based on the measured results. Therefore, a signal level which is reduced by the band-elimination filter is recovered.
In accordance with such a conventional technique, it is possible to achieve low noise and low distortion, but there is a problem in which a frequency of signals measured is limited by frequency characteristics of the band-elimination filter. There is a possibility in which it is possible to measure with regard to a wide frequency range if multiple band-elimination filters with different frequency characteristics are applied. However, there is another problem in which a size of a circuit increases along with an increase of a number of band-elimination filters. Moreover, in the above-described conventional technique, there is a precondition in which the band-elimination filter never generates distortions. However, in practical cases, there is a problem in which it is difficult to produce a band-elimination filter which generates low distortion.